The dye-sensitized solar cells (DSSCs) were originally developed by M. Gratzel, hence they are also called the Gratzel cells. Essentially, a DSSC is comprised of a photoanode, electrolyte, and platinum electrodes. The photoanode is basically constructed of a transparent and conductive glass substrate having a conductive surface of a layer of ITO or FTO over the glass substrate, a semiconductor film having titanium dioxide (TiO2) particles on the conductive surface of the conductive glass substrate, and photosensitizing dye adsorbed onto the semiconductor film.
Currently, a photosensitizing dye can be adsorbed onto the photoanode of a DSSC by using either the method of conventional impregnation (atmospheric pressure) or the method of supercritical fluid impregnation.
For the method of conventional impregnation, the rate at which a photosensitizing dye solution enters into pores is rendered slower as it is affected by the viscosity and surface tension of the photosensitizing dye solution. As for the method of supercritical fluid impregnation, highly pressurized CO2 is used as a solvent, and a co-solvent is also added thereinto to increase the solubility thereof, because the solubility of photosensitizing dye in the highly pressurized CO2 is generally low. Consequently, the highly pressurized CO2 and the co-solvent are combined to give rise to a mixed fluid, which possesses lower surface tension and viscosity than the solvent used in the conventional impregnation, and thus encounters less mass transfer resistance when photosensitizing dye molecules diffuse into the pores. However, the solubility of organic dyes in supercritical fluids even with the co-solvent are still low, which means the solution contains low saturated concentration of photosensitizing dye, thereby undermining the effects of impregnation.
In other words, the search for a process for preparing a DSSC photoanode with high photosensitizing dye adsorption is still being continuously undertaken by the industry.